Electroplating bath and process for maintaining plated alloy composition stable

ABSTRACT

A plating bath and process for electroplating coatings of palladium nickel alloys on a conductive substrate at current densities in the range of 10 amps/sq. ft. to 150 amps/sq. ft. wherein the palladium content of the alloy remains substantially constant despite current density variations during plating. The alloy composition stability is achieved by adding at least about 15 parts per million of iodate ions to the plating bath.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 880,872 filedJuly 1, 1986 and now U.S. Pat. No. 4,743,346.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to coatings of electroplated palladium-nickelalloys and in particular to a plating bath process for controlling thestability of the alloy composition in such coating over a wide variationof electroplating current densities.

2. Description of the Prior Art

Gold has historically been the plating material of choice for electricalcontacts because of its resistance to corrosion, good solderabilityproperties and low electrical contact resistance at low loads. Sincegold platings are expensive, lower cost substitutes have been sought.

Alloys of palladium-nickel have shown much promise as a gold substitutefor plating electrical contacts. One of the more successful suchpalladium-nickel coatings is described in U.S. Pat. No. 4,463,060granted July 31, 1984 and assigned to the same assignee as the presentinvention, the disclosure of which is hereby incorporated herein byreference. The palladium-nickel electroplated surface coating describedin this patent effectively protects the substrate from corrosion, ispermanently solderable and exhibits reduced electrical contactresistance at low loads.

The coatings in the aforenoted U.S. Pat. No. 4,463,060 patent areprepared by electroplating in a bath of palladium (II) ammine chloride,nickel ammine sulfate, a small amount of brighteners, and a conductivesalt. Electroplating is carried out at a current density ranging fromabout 5 to 25 amps/sq.dm., or 50 to 250 amps/sq.ft. (asf). At currentdensities in the upper portion of this range, above about 100 asf, thePd-Ni composition of the plated coating can be fairly readilycontrolled. As current densities decrease below this level, controllingthe alloy composition becomes increasingly difficult.

Controlling the Pd-Ni alloy composition during electroplating isextremely important. The properties of Pd-Ni alloy coatings which areimportant for electronic connector applications, such as solderability,ductility, hardness, thermal stability of contact resistance andenvironmental corrosion resistance, vary significantly with fluctuationin electroplated alloy composition. Accurate control of the level andconstancy of plated Pd-Ni alloy composition is therefore necessary toassure the desired properties of connector products.

Other plated connector performance criteria, such as absence of porosityand wear life, are strong functions of the precious metal alloythickness. All widely used, nondestructive, production techniques formeasurement of precious metal alloy coating thickness, such as electronbackscatter spectroscopy and x-ray fluorescence, must be standardizedfor a specific alloy composition. Therefore, in the production of Pd-Nialloy plated connector products, the stability of the alloy compositionis also necessary to achieve the desired control of alloy coatingthickness and the properties that are thickness-dependent.

Of particular concern is the stability of the alloy composition as afunction of current density. In the commercial plating of formedterminals, there can be variations in current density as high as afactor of four depending upon location on the connector. The magnitudeof the current density variation is dependent upon the part geometry,the plating cell design, and other factors. A typical range of currentdensities for most formed terminals is 25-100 asf. Locations on a fewterminals might be plated as low as 10 asf or as high as 150 asf.

The effects of current density variation upon alloy compositionstability can be better explained with reference to FIG. 1 and Examples1-3 below. For the purpose of this invention, the stability parameterfor evaluation of Pd-Ni alloy plating process performance is defined asthe difference between the Pd content in weight percent of an alloydeposited at 100 asf and that for an alloy deposited at 25 asf. Thisdifference, which is illustrated for Curve A on FIG. 1, will be referredto and indicated by the symbol Δ Wt% Pd.sub.(100-25). Plating bathsformulated with typical commercially available palladium ammine chloridesalts and organic brightener systems have a Δ Wt% Pd.sub.(100-25) in theapproximate range of 12 to 22 as shown in Examples 1, 2, and 3. InExample 1, identical plating runs with respect to bath chemistry andplating conditions were conducted with palladous ammine dichloride saltsfrom six different commercial sources. The Δ Wt% Pd.sub.(100-25) forthese runs ranged from 13.0 to 18.7, indicating that they were allunstable with respect to the desired constancy of alloy composition.

SUMMARY OF THE INVENTION

In the preferred embodiments of this invention, the palladium-nickelalloy plating baths have Δ Wt% Pd.sub.(100-25) 's in the range of 0 to6. This is based upon the discovery that the intentional addition ofiodide ions to palladium-nickel alloy plating baths results in apronounced improvement in process stability as indicated by asignificant decrease in the Δ Wt% Pd.sub.(100-25) parameter. It has alsobeen discovered that the intentional addition of iodate ions to suchbaths will likewise improve the process stability. Purification ofpalladium salts to remove certain chemical species that promoteinstability is also important and is necessary to achieve the ultimatein stability [Δ Wt% Pd.sub.(100-25) =0].

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the effects of current density variationupon Pd-Ni alloy composition stability, wherein Curve A representsplating run V of Example 1 and Curve B illustrates the beneficialeffects of a 15 ppm iodide addition according to the present inventionas demonstrated by Example 4.

FIG. 2 is a graph illustrating the Pd-Ni alloy stability as a functionof the iodide ion concentration used in the plating run of Example 5.

FIG. 3 is a schematic illustrating a possible mechanism at the electrodeinterface which may contribute to the constancy effect on the Pd-Nialloy composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As noted above, the addition of iodide or iodate ions to thepalladium-nickel alloy plating bath has a pronounced effect oncontrolling the plated alloy composition. This is demonstrated by asignificant decrease in the Δ Wt% Pd.sub.(100-25) parameter.

For palladium-nickel alloy plating baths that do not contain organicbrighteners, the addition of as little as 15 ppm of iodide ion canresult in a Δ Wt% Pd.sub.(100-25) of 2.0, as demonstrated in Example 4,below. The alloy composition versus current density plot for the 15 ppmrun in Example 4 appears as Curve B in FIG. 1. Curve A in FIG. 1 is foran identical plating run in terms of process chemistry and operatingconditions except that a sodium vinyl sulfonate brightener was usedinstead of iodide ions (see Example 1, Plating Run V). The replacementof iodide ions with sodium vinyl sulfonate resulted in an increase in ΔWt% Pd.sub.(100-25) to 18.7.

Iodide ions act as a brightener when added to a palladium-nickel alloyplating bath containing no organic additives. The iodide ion additionnot only results in the plating of a mirror bright coating, but it alsoincreases the maximum current density for the deposition of smooth,dense, nonporous coatings.

Additions of small amounts of iodate ions will also significantly reduceΔ Wt% Pd.sub.(100-25). For example, the addition of as little as 15 ppmof iodate ion to a palladium-nickel alloy plating bath containing anorganic brightener such as sodium vinyl sulfonate as described inExample 16 will reduce the Δ Wt% Pd.sub.(100-25) stability parameterfrom 16.7 to 7.8. When the amount of iodate ions added is raised to 100ppm, the stability parameter is reduced further to 4.2 in Example 16.When no organic brightener is used, such as in the bath of Example 17,additions of 15 ppm and 100 ppm of iodate ions reduce the Δ Wt%Pd.sub.(100-25) stability parameter to 7.6 and 7.8, respectively.Moreover, as with iodide ion addition, the iodate ions also appear toact as a brightener since the coatings of Example 17 all have a bright,mirror-like appearance.

For palladium-nickel alloy plating processes based on certian organicbrighteners, such as aliphatic sulfonic acids, the addition of smallamounts of iodide ion is very effective in promoting process stability.This fact is illustrated in Example 5, which shows the effect of iodideion additions ranging from 6 to 100 ppm on the constancy of alloycomposition plated by a process based on a sodium vinyl sulfonatebrightener. The stability parameter for runs in Example 5 plotted versusiodide ion concentration appears in FIG. 2. These data indicate that anaddition of only 25 ppm of iodide ions was sufficient to decrease Δ Wt%Pd.sub.(100-25) from 18.7 to a plateau level of about 5.

For palladium-nickel alloy plating processes based on other organicbrighteners such as quaternized pyridines, larger additions of iodideions are required to improve process stability. This is illustrated byExample 6, which shows the effect of iodide ion additions ranging from23 to 300 ppm on the constancy of alloy composition plated by a processbased on technical grade N-benzyl niacin internal salt as brightener.This salt is CAS Registry No. 15990-43-3-pyridinium3-carboxy-1-(phenylmethyl)hydroxide inner salt, and will be referred tosubsequently as "pyridinium salt." The addition of 300 ppm of iodide iondecreased the Δ Wt% Pd.sub.(100-25) for this process from 16.9 to 11.7.Additional improvements in the constancy of the alloy composition platedby this process could be achieved by decreasing the concentration of"pyridinium salt" and by the removal of impurities in the brightener,some of which might be promoting instability.

The effect of palladium salt purification is shown by Examples 7, 8 and9, below. Palladium salts used in Examples 7, 8 and 9 were purified byutilizing the fact that palladium diammine chloride, Pd(NH₃)₂ Cl₂, isinsoluble in water and will form a precipitate when a solution ofpalladium tetrammine chloride is treated with an excess of hydrochloricacid, as per the following reaction: ##STR1## Ammonia is liberated andchloropalladous acid is formed.

Palladium diammine chloride, however, can be solubilized by treatingwith ammonia (dissolving in NH₄ OH), as follows:

    Pd(NH.sub.3).sub.2 Cl.sub.2 +2NH.sub.3 →Pd(NH.sub.3).sub.4 Cl.sub.2

Palladium tetrammine chloride salt is readily solubilized in water.

Thus, to purify further a purchased palladium tetrammine salt, thefollowing procedure was used:

(a) Dissolve the palladium tetrammine salt, Pd(NH₃)₄ Cl₂, in deionizedwater.

(b) Precipitate the palladium diammine chloride, Pd(NH₃)₂ Cl₂, by addingan excess of hydrochloric acid.

(c) Filter the precipitate from the mother liquor and wash several timeswith deionized water.

(d) Redissolve the precipitated palladium diammine chloride in NH₄ OH,again forming a solution of palladium tetrammine chloride.

To purify further a purchased palladium diammine chloride salt, theprocedure was as follows:

(a) Dissolve the palladium diammine chloride salt in NH₄ OH, forming asolution of the palladium tetrammine chloride.

(b) Precipitate the diammine chloride by adding an excess ofhydrochloric acid.

(c) Filter the precipitated palladium diammine from the mother liquorand wash several times with deionized water.

Based on the above, one cycle of purification is defined as the seriesof steps which will repeat the chemical identity of the original entitytreated (e.g., palladium diammine chloride back to palladium diamminechloride). A palladium balance made on this series of steps verified theabove stoichiometry.

It is well known to those skilled in the art of chemical synthesis thatprecipitation (recrystallization) will tend to purify the precipitatedproduct, rejecting impurities to the supernatant mother liquor. Besidesthis method of purification, other methods of purification will suggestthemselves to those skilled in the art, such as reaction of an aqueoussolution of the palladium salt with hydrogen peroxide (especially fororganic impurities), or passing the palladium solution through a bed ofpalladium powder, carbon treatment, etc. It should be noted that somepurification methods may also remove the iodide ions as well as theundesired impurities which promote instability. In such cases, iodideions must be added to the requisite concentration after purification.Strict control of the impurity content of all other ingredients of thepalladium-nickel bath is also necessary.

Examples 8 and 9 also show the powerful effect of purification byprecipitation as described above combined with the addition of iodide tothe palladium-nickel plating bath. A one-cycle purification of apalladous tetrammine chloride salt, formulated into a plating bath withthe addition of 31 ppm iodide ion resulted in a ΔPd.sub.(100-25) of 4.2.When a similar plating bath was formulated with a palladous tetramminechloride salt purified through two cycles of purification (35 ppm iodideadded) ΔPd.sub.(100-25) was 0.4, essentially a constant alloycomposition over the current density range of 25 asf to 100 asf.

The present invention has broad applicability with respect to allpalladium-nickel alloy plating processes. The effectiveness of iodideadditions in establishing constancy of plated alloy composition in therange of current densities from 25 to 100 amps/sq.ft. has beendemonstrated for a variety of nickel salt types (see Example 10),different conductive salts (see Example 11), a broad range of agitationlevels (see Example 12), and a broad range of Pd/Ni molar concentrationratios which result in the deposition of a broad range of alloycompositions (see Examples 13, 14 and 15). Also, the iodide additionappears effective over the typical pH range of about 7-9 normallyemployed in commercial plating baths.

The elements of a possible mechanism to explain this constancy effect onalloy composition are shown in FIG. 3. During electroplating, theconnector terminal acts as a solid cathode electrode to which thepalladium-nickel alloy is to be electroplated. An adsorbed surface layerof iodide ion forms an effective "bridge" for the palladium ion in thebath, probably the Pd(NH₃)₄ ++ ion, to transfer charges to theelectrode. The iodide ion, however, does not offer an effective "bridge"to the nickel ion species. This "ligand-bridging" effect has beendescribed in the literature. In essence, such a "bridge" eases thetransfer of charge to or from the target ion (in this case, thepalladium ion) by both adsorbing on the electrode and also insertingitself into the coordinating sphere of the target ion.

This effect, which is a differential one in that, as stated above, theiodide ion does not offer as effective a bridge for the nickel speciesas it does for the palladium species, might be written as follows:##STR2## In the above representation, M is the metal cathode electrode,X⁻⁻ is iodide ion used to obtain the compositional constancy effect andthe entity in the brackets represents the bridge formed by the iodideion between the metal and the palladium species. The kind ofcoordination represented here is sometimes referred to an "inner sphere"mechanism because at least one ligand is shared, that is, it belongssimultaneously to both coordination shells.

It should also be noted that irrespective of whether one initially addsiodide ions or iodate ions, the electrochemical reactions at the anodeand the cathode will continuously recycle these ions. For example, theiodate ion is reduced under cathodic conditions to iodide ion:

    IO.sub.3.sup.-- +3H.sub.2 O+6e.sup.-- →I.sup.-- +60H.sup.--

At the anode, the reaction is reversed:

    I.sup.-- +60H.sup.-- →IO.sub.3.sup.-- +3H.sub.2 O+6e.sup.--

What is important is that iodide ions are available for adsorption atthe cathode which is the connector terminal being plated.

Adsorption of the iodide ion will be facilitated if the cathode is at apotential more positive than its point of zero charge (PZC). At morepositive potentials than the PZC, the electrode surface has a netpositive charge; at more negative potentials than the PZC, it has a netnegative charge.

The above described mechanism is consistent with the observed role ofpurification in enhancing this effect (see Examples 7, 8 and 9).Purification, as practiced in the examples in this patent, would tend toremove adsorbable substances from the bath (by virtue of their removalfrom the palladium source) which might compete with iodide (say,surface-active agents) for sites at the electrode but which would notoffer a bridge to the palladium. It is apparent that differentsubstances will vary greatly in the extent to which they will competewith iodide for sites at the surface. Thus, the presently acceptedtheories of the action of surface-active agents would suggest that morehydrophobic agents would compete more effectively for sites at anelectrode surface immersed in an aqueous solution and thus tend todiminish the effectiveness of iodide in maintaining compositionalconstancy. This is seen in the results for the "pyridinium salt" inwhich a quantity of this substance in the bath which is far less, on amolar basis, than typical additions of sodium vinyl sulfonate, reducesthe effectiveness of the iodide (see Example 6). In fact, itsdeleterious effect could not be completely overcome by substantialadditions of iodide. This finding also indicates that a surface effectis involved since, if the effect were a bulk effect, constancydestabilization induced by the quaternized pyridine should have been"neutralized" by the large additions of iodide. In general, the degreeof hydrophobicity can be correlated with the relative quantity oforganic character in the molecule in question. Where there is nocompetition at all from deliberately added substances, the iodide ionalone is more effective in maintaining composition constancy than in thepresence of sodium vinyl sulfonate (see Example 4).

Another key property of surface-active agents, aside from their degreeof hydrophobicity, which could have a profound effect on theircompetition with iodide, would be the charge on the surface-activemoiety; a positively-charged active moiety might neutralize the iodideeffect by creating competition between itself and the palladium ion foriodide.

Since this model depends on at least a surface layer of coverage ofiodide on the cathode electrode surface, the optimum concentration ofiodide in the bulk solution (whether due initially to addition of iodideions, iodate ions, or both) may differ under differing conditions ofplating, mass transfer, etc.

EXAMPLES

The following specific examples describe the invention in greaterdetail. All examples were carried out on copper alloy disks that hadbeen subjected to conventional preplate treatments as practiced in theart. The disks were then electroplated with a pure nickel coating by aconventional nickel sulfamate plating process. The nickel undercoatprevents copper contamination of certain palladium-nickel plating baths,but it is not necessary to the practice of the invention. Thenickel-plated surfaces were activated by immersion in a 20 volume %solution of sulfuric acid prior to palladium-nickel alloy plating. Boththe nickel plating and the palladium-nickel alloy plating steps wereconducted in a conventional rotating-disk-electrode plating apparatus atspeeds of rotation ranging from 100 to 500 rpm.

The palladium-nickel alloy coating thicknesses were 60 μin. which issufficient to permit accurate composition analysis using anEnergy-Dispersive X-ray Analysis (EDXA) technique with an acceleratingvoltage for exciting electrons of 20 kV.

EXAMPLE 1

This example illustrates the typical instability of the composition ofpalladium-nickel alloys deposited by a process formulated with palladiumsalts obtained from a variety of commercial sources. Plating bathshaving the same composition were formulated with palladium salts fromsix different sources as described in Table I.

                  TABLE I                                                         ______________________________________                                        Plating  Palladium-Salt Description                                           Run      Source                                                               Code     Code          Salt Type                                              ______________________________________                                        I        A             Tetrammine dichloride                                  II       B             Tetrammine dichloride                                  III      C             Diammine dichloride                                    IV       D             Diammine dichloride                                    V        E             Tetrammine dichloride                                  VI       F             Diammine dichloride                                    ______________________________________                                    

The plating process composition and plating conditions were as follows:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:  20 g/l from salts cited                                                       in Table I                                                 Ni concentration:  10 g/l as nickel ammine                                                       sulfate                                                    Sodium vinyl sulfonate:                                                                          2.8 g/l                                                    Ammonium sulfate:  50 g/l                                                     Ammonium hydroxide:                                                                              Quantity sufficient to                                                        achieve desired pH.                                        Plating Conditions                                                            Temperature:       48° C.                                              pH:                8.5 (adjusted by addition                                                     of NH.sub.4 OH or HCl)                                     Speed of disk rotation:                                                                          500 rpm                                                    ______________________________________                                    

Disks were plated with palladium-nickel alloy coatings at currentdensities ranging from 25 to 200 amp/sq.ft. from six baths of thecomposition cited above, each bath being formulated with one of the sixdifferent palladium salts cited in Table I. The results of coating alloycompositions analyses appear in Table II. The alloy compositionstability parameter [Δ Wt% Pd.sub.(100-25) ] for the six plating bathsranged from 13.0 to 18.7.

                  TABLE II                                                        ______________________________________                                        Plated Pd--Ni Alloy Composition                                               in Weight % Pd* at Indicated                                                  Current Density                                                               Plating                                                                       Run                                                                           Code   25 asf  50 asf  75 asf                                                                              100 asf                                                                             Δ Wt % Pd.sub.(100-25)               ______________________________________                                        I      67.3    77.3    --    84.3  17.0                                       II     74.3    80.8    --    87.3  13.0                                       III    69.5    77.4    80.3  83.8  14.3                                       IV     66.3    76.2    79.8  83.4  17.1                                       V      63.4    75.0    --    82.1  18.7                                       VI     62.4    66.4    75.6  80.2  17.8                                       ______________________________________                                         *Note: Balance, nickel.                                                  

EXAMPLE 2

Palladium-nickel alloy coatings were electrodeposited on disks atcurrent densities ranging from 25 to 200 asf using the bath chemistryand plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:  17.0 g/l as palladous                                                         tetrammine dichloride                                      Ni concentration:  11.0 g/l as nickel ammine                                                     chloride                                                   Sodium vinyl sulfonate:                                                                          2.8 g/l                                                    Ammonium sulfate:  50 g/l                                                     Ammonium hydroxide:                                                                              Quantity sufficient to                                                        achieve desired pH.                                        Plating Conditions                                                            Temperature:       48° C.                                              pH:                8.0                                                        Speed of rotation: 500 rpm                                                    ______________________________________                                    

Results for coating composition analyses as a function of currentdensity appear in Table III. The process had a Δ Wt% Pd.sub.(100-25)parameter of 21.4.

                  TABLE III                                                       ______________________________________                                        Current        Pd--Ni Alloy                                                   Density        Composition                                                    (asf)          Wt % Pd  Wt % Ni                                               ______________________________________                                        25             47.3     52.7                                                  50             54.9     45.1                                                  75             62.5     37.5                                                  100            68.7     31.3                                                  200            77.0     23.0                                                  ______________________________________                                    

EXAMPLE 3

Palladium-nickel alloy coatings were electrodeposited on disks atcurrent densities ranging from 25 to ®asf using the bath chemistries andplating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:  15.0 g/l as palladous                                                         tetrammine dichloride                                      Ni concentration:  7.5 g/l as nickel chloride                                 "Pyridinium salt": 0.6 g/l                                                    Ammonium chloride: 30 g/l                                                     Ammonium hydroxide:                                                                              Quantity sufficient to                                                        achieve desired pH.                                        Planting Conditions                                                           Temperature:       48° C.                                              pH:                8.5                                                        Speed of rotation: 500 rpm                                                    ______________________________________                                    

Results for coating composition analyses as a function of currentdensity appear in Table IV. The process had a Δ Wt% Pd.sub.(100-25) of16.9.

                  TABLE IV                                                        ______________________________________                                        Current        Pd--Ni Alloy                                                   Density        Composition                                                    (asf)          Wt % Pd  Wt % Ni                                               ______________________________________                                        25             64.5     35.5                                                  50             72.1     27.9                                                  75             77.4     22.6                                                  100            81.4     18.6                                                  200            84.2     15.8                                                  ______________________________________                                    

EXAMPLE 4

This example illustrates the beneficial effect of iodide ion addition toa palladium-nickel alloy plating bath significantly improving theconstancy of alloy composition. Palladium-nickel alloy coatings wereelectrodeposited on disks at current densities ranging from 25 to 200asf from plating baths containing 15 and 50 ppm of iodide ions, and from10 to 200 asf from a plating bath containing 25 ppm iodide. The basicbath chemistry and plating conditions were as follows:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:   20 g/l as palladous                                                           tetrammine dichloride                                     Ni concentration:   10 g/l as nickel ammine                                                       sulfate                                                   Ammonium sulfate:   50 g/l                                                    Ammonium hydroxide: Quantity sufficient to                                                        achieve desired pH.                                       Plating Conditions                                                            Temperature:        48° C.                                             pH:                 8.5                                                       Speed of rotation:  500 rpm                                                   ______________________________________                                    

Coating composition analyses as a function of current density and iodideion concentration level appear in Table V. The plating bath containing15 ppm iodide had a Δ Wt% Pd.sub.(100-25) of 2.

                  TABLE V                                                         ______________________________________                                        Plated Pd--Ni Alloy Composition                                               in Weight % Pd* at Indicated                                                  Current Density                                                               Iodide Ion                                                                    Concentration                                                                          10     25     50   75   100  200  Δ Wt %                       ppm      asf    asf    asf  asf  asf  asf  Pd.sub.(100-25)                    ______________________________________                                        15       --     90.1   91.3 91.5 92.1 89.1 2.0                                25       87.2   89.8   90.8 --   91.7 87.8 1.9                                50       --     90.9   91.8 92.2 92.6 89.7 1.7                                ______________________________________                                         *Note: Balance, nickel.                                                  

EXAMPLE 5

This example illustrates the beneficial effect of iodide ion addition toa palladium-nickel alloy plating bath containing sodium vinyl sulfonatein significantly improving the constancy of alloy composition.Palladium-nickel coatings were electrodeposited on disks at currentdensities ranging from 25 to 100 asf from plating baths containing 0, 6,15, 25, 50 and 100 ppm of iodide ion. The basic bath chemistry andplating conditions were as follows:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:  20 g/l as palladous                                                           tetrammine dichloride                                      Ni concentration:  10 g/l as nickel ammine                                                       sulfate                                                    Sodium vinyl sulfonate:                                                                          2.8 g/l                                                    Ammonium hydroxide:                                                                              Quantity sufficient to                                                        achieve desired pH.                                        Plating Conditions                                                            Temperature:       48° C.                                              pH:                8.5                                                        Speed of rotation: 500 rpm                                                    ______________________________________                                    

The stability parameters [Δ Wt% Pd.sub.(100-25) ] determined as afunction of iodide ion concentration level appear in Table VI. Theaddition of 25 ppm of iodide ion decreased the Δ Wt% Pd.sub.(100-25)from 18.7 to 5.2.

                  TABLE VI                                                        ______________________________________                                        Iodide Ion                                                                    Concentration  Δ Wt %                                                   ppm            Pd.sub.(100-25)                                                ______________________________________                                         0             18.7                                                            6             12.8                                                           15             7.6                                                            25             5.2                                                            50             5.0                                                            100            5.9                                                            ______________________________________                                    

EXAMPLE 6

This example illustrates the beneficial effect of iodide ion addition toa palladium-nickel alloy plating bath containing a quaternized pyridinein improving the constancy of alloy composition. Palladium-nickelcoatings were electrodeposited on disks at current densities rangingfrom 25 to 100 asf from plating baths containing 0, 23, 100 and 300 ppmof iodide ion. The basic bath chemistry and plating conditions were asfollows:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:  15.0 g/l as palladous                                                         tetrammine dichloride                                      Ni concentration:  7.5 g/l as nickel chloride                                 "Pyridinium salt"  0.6 g/l                                                    Ammonium hydroxide:                                                                              Quantity sufficient to                                                        achieve desired pH.                                        Plating Conditions                                                            Temperature:       48° C.                                              pH:                8.5                                                        Speed of rotation: 500 rpm                                                    ______________________________________                                    

The stability parameters [Δ Wt% Pd.sub.(100-25) ] determined as afunction of iodide ion concentration appear in Table VII. The additionof 300 ppm of iodide ion decreased the Δ Wt% Pd.sub.(100-25) from 16.9to 11.7.

                  TABLE VII                                                       ______________________________________                                        Iodide Ion                                                                    Concentration  Δ Wt %                                                   ppm            Pd.sub.(100-25)                                                ______________________________________                                         0             16.9                                                            23            15.5                                                           100            13.9                                                           300            11.7                                                           ______________________________________                                    

EXAMPLE 7

This example illustrates the beneficial effect of palladium saltpurification in improving the constancy of composition ofelectrodeposited palladium-nickel alloys. Part of a shipment of a lot ofcommercially available palladous tetrammine dichloride salts waspurified by one recrystallization cycle as described above.Palladium-nickel alloy coatings were electrodeposited on disks atcurrent densities ranging from 25 to 100 asf from a bath formulated withthe as-received palladium salt and a bath of identical basic chemistryformulated with the purified palladium salts under the same platingconditions. The basic bath chemistry and plating conditions wereidentical to those for Example 1. The iodide ion concentrations for thebaths were <1 ppm. The process formulated with the as-received palladiumsalt had a stability parameter of 18.7 whereas that formulated with thepurified salt had a stability parameter of 14.5.

EXAMPLE 8

This example illustrates the beneficial effect of iodide ion additionand palladium salt purification on the constancy of the composition ofelectrodeposited palladium-nickel alloys. A sample of palladoustetrammine chloride salt was purified through one recrystallizationcycle as described earlier. Palladium-nickel alloys were plated on disksat current densities ranging from 25 to 100 asf using a bath chemistryand plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             20 g/l                                                        Ni concentration:                                                                             10 g/l as nickel ammine                                                       sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Iodide ion:     31 ppm                                                        Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           Quantity sufficient to                                                        achieve desired pH.                                           Plating Conditions                                                            Temperature:    48° C.                                                 pH:             8.5                                                           Speed of rotation:                                                                            500 rpm                                                       ______________________________________                                         The Δ Wt % Pd.sub.(10025) for the process was 4.2.                 

EXAMPLE 9

This example illustrates the beneficial effect of iodide ion additionand palladium salt purification on the constancy of the composition ofelectrodeposited palladium-nickel alloys. A sample of palladoustetrammine chloride salt was purified through two recrystallizationcycles as described earlier. Palladium-nickel alloys were plated ondisks at current densities ranging from 25 to 100 asf using a bathchemistry and plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             20 g/l                                                        Ni concentration:                                                                             10 g/l as nickel ammine                                                       sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Iodide ion:     35 ppm                                                        Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           Quantity sufficient to                                                        achieve desired pH.                                           Plating Conditions                                                            Temperature:    48° C.                                                 pH:             8.5                                                           Speed of rotation:                                                                            500 rpm                                                       ______________________________________                                         The Δ Wt % Pd.sub.(10025) for the process was 0.4.                 

EXAMPLE 10

Palladium-nickel alloy coatings were electrodeposited on disks atcurrent densities ranging from 25 to 100 asf under identical operatingconditions from baths that were formulated with three different types ofnickel salts, an ammine sulfate, a sulfate, and a chloride. Thepalladium salt, other basic process chemistry parameters, and platingconditions were identical to those for Example 8. The constancy ofpalladium alloy composition for the three different types of nickel saltappear in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        Nickel Salt                                                                   Type           Δ Wt % PD.sub.(100-25)                                   ______________________________________                                        Ammine sulfate 4.2                                                            Sulfate        2.8                                                            Chloride       2.3                                                            ______________________________________                                    

EXAMPLE 11

Palladium-nickel alloy coatings were electrodeposited on disks atcurrent densities ranging from 25 to 100 asf under identical operatingconditions from baths that were formulated with two different types ofconductive salts. The palladium salt, other basic process chemistryparameters, and plating conditions were identical to those for Example8. The constancy of palladium alloy composition for the two differenttypes of conductive salts appear in Table IX.

                  TABLE IX                                                        ______________________________________                                        Conductive Salt                                                               Type             Δ Wt % Pd.sub.(100-25)                                 ______________________________________                                        Ammonium sulfate 4.2                                                          Ammonium Chloride                                                                              0.8                                                          ______________________________________                                    

EXAMPLE 12

This example illustrates the effectiveness of iodide ion additions inachieving constancy of palladium-nickel alloy composition as a functionof current density for a range of agitation levels. Palladium-nickelalloys were plated on disks rotated at speeds of 100 and 500 rpm using abath chemistry and plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             20 g/l as palladous                                                           diammine dichloride                                           Ni concentration:                                                                             l0 g/l as nickel ammine                                                       sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Iodide ion conc.:                                                                             31 ppm                                                        Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           Quantity sufficient to                                                        achieve desired pH.                                           Plating Conditions                                                            Temperature:    48° C.                                                 pH:             8.6                                                           Current Density:                                                                              25 asf and 100 asf                                            ______________________________________                                         For a speed of rotation of 100 rpm, the Δ Wt % Pd.sub.(10025) was       3.2. For a speed of rotation of 500 rpm, the Δ Wt % Pd.sub.(10025)      was 2.8.                                                                 

EXAMPLE 13

This example illustrates the beneficial effects of iodide ion additionin improving the constancy of the composition of palladium-nickel alloyselectrodeposited from a bath having palladium-to-nickel molarconcentration ratios of 0.86. Palladium-nickel alloys were plated ondisks at current densities ranging from 25 to 100 asf using a bathchemistry and plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             17.0 g/l as palladous                                                         diammine dichloride                                           Ni concentration:                                                                             11.0 g/l as nickel ammine                                                     sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           Quantity sufficient to                                                        achieve desired pH.                                           Platinq Conditions                                                            Temperature:    48° C.                                                 pH:             8.0                                                           Speed of rotation:                                                                            500 rpm                                                       ______________________________________                                    

At the completion of the initial plating run, 100 ppm of iodide ion wasadded to the plating bath and a second set of disks was plated withpalladium-nickel alloys at the same current density settings. Theresults of coating alloy composition analyses for the runs before andafter the addition of iodide ions appear in Table X.

                  TABLE X                                                         ______________________________________                                        Bath         Plating Pd--Ni Alloy Composition                                 Iodide Ion   in Weight % Pd* at Indicated                                     Concentration,                                                                             Current Density                                                  ppm          25 asf  50 asf    75 asf                                                                              100 asf                                  ______________________________________                                         0           47.3    54.9      62.5  68.7                                     100          75.8    77.1      76.5  77.3                                     ______________________________________                                         *Note: balance, nickel.                                                  

The presence of 100 ppm of iodide ion in the plating bath decreased theΔ Wt% Pd.sub.(100-25) from 21.4 to 1.5.

EXAMPLE 14

This example illustrates the beneficial effects of iodide ion additionin improving the constancy of the composition of palladium-nickel alloyselectrodeposited from a bath having palladium-to-nickel molarconcentration ratio of 0.55. Palladium-nickel alloys were plated ondisks at current densities ranging from 25 to 100 asf using a bathchemistry and plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             15.6 g/l as palladous                                                         diammine dichloride                                           Ni concentration:                                                                             15.4 g/l as nickel ammine                                                     sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           Quantity sufficient to                                                        achieve desired pH.                                           Plating Conditions                                                            Temperature:    48° C.                                                 pH:             8.5                                                           Speed of rotation:                                                                            500 rpm                                                       ______________________________________                                    

At the completion of the initial plating run, 100 ppm of iodide ion wasadded to the plating bath and a second set of disks was plated withpalladium-nickel alloys at the same current density settings. Theresults of coating alloy composition analyses for the runs before andafter the addition of iodide ions appear in Table XI.

                  TABLE XI                                                        ______________________________________                                        Bath         Plating Pd--Ni Alloy Composition                                 Iodide Ion   in Weight % Pd* at Indicated                                     Concentration,                                                                             Current Density                                                  ppm          25 asf  50 asf    75 asf                                                                              100 asf                                  ______________________________________                                         0           48.4    56.7      63.2  66.5                                     100          68.4    66.9      65.0  70.9                                     ______________________________________                                         *Note: balance, nickel.                                                       The presence of 100 ppm of iodide ion in the plating bath decreased the       Δ Wt % Pd.sub.(10025) from 18.1 to 2.5.                            

EXAMPLE 15

This example illustrates the beneficial effects of iodide ion additionin improving the constancy of the composition of palladium-nickel alloyselectrodeposited from a bath having a low palladium-to-nickel molarconcentration ratio of 0.24. Palladium-nickel alloys were plated ondisks at current densities ranging from 25 to 100 asf using a bathchemistry and plating conditions set forth below:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             7.4 g/l as palladous                                                          diammine dichloride                                           Ni concentration:                                                                             17.0 g/l as nickel ammine                                                     sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Iodide ion conc.:                                                                             11 ppm                                                        Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           Quantity sufficient to                                                        achieve desired pH.                                           Plating Conditions                                                            Temperature:    48° C.                                                 pH:             8.0                                                           Speed of rotation:                                                                            500 rpm                                                       ______________________________________                                    

At the completion of the initial plating run, 89 ppm of iodide ion wasadded to the plating bath and a second set of disks was plated withpalladium-nickel alloys at the same current density settings. Theresults of coating alloy composition analyses for the runs before andafter the addition of iodide ions appear in Table XII.

                  TABLE XII                                                       ______________________________________                                        Bath         Plating Pd--Ni Alloy Composition                                 Iodide Ion   in Weight % Pd* at Indicated                                     Concentration,                                                                             Current Density                                                  ppm          25 asf  50 asf    75 asf                                                                              100 asf                                  ______________________________________                                         11          20 6    21.2      28.3  38.5                                     100          33.6    35.1      37.8  43.0                                     ______________________________________                                         Note: balance, nickel.                                                   

The addition of 89 ppm of iodide ion in the plating bath decreased the ΔWt% Pd.sub.(100-25) from 17.9 to 9.4.

EXAMPLE 16

This example illustrates the beneficial effect of iodate ion additionsin significantly improving the constancy of palladium-nickel alloycomposition electrodeposited from baths containing sodium vinylsulfonate as an organic brightener. All baths were formulated with thesame lot of a commercially available palladium diammine dichloride salt.Palladium-nickel alloy coatings were electrodeposited onto disks atcurrent densities ranging from 25 to 100 asf from baths containing 0, 15and 100 ppm of iodate ions added as the sodium salt. The basic chemistryand plating conditions were as follows:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             20 g/l as palladium                                                           diammine dichloride                                           Ni concentration:                                                                             10 g/l as nickel ammine                                                       sulfate                                                       Sodium vinyl sulfonate:                                                                       2.8 g/l                                                       Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           quantity sufficient to                                                        achieve desired pH                                            Plating Conditions                                                            Temperature:    48° C.                                                 pH:             8.5                                                           Speed of disk rotation:                                                                       500 rpm                                                       ______________________________________                                    

Coating composition analyses as a function of current density and iodateion addition level appear in Table XIII. The process without an additionof iodate ions had a Δ Wt% Pd.sub.(100-25) stability parameter of 16.7;whereas, the Δ Wt% Pd.sub.(100-25) for the process with 100 ppm ofiodate ions was only 4.2.

                  TABLE XIII                                                      ______________________________________                                        Iodate    Plated Pd--Ni Alloy Composition                                     Ion       in Weight % Pd* at Indicated                                        Addition  Current Density                                                     ppm       25 asf  50 asf    100 asf                                                                             Δ Wt % Pd                             ______________________________________                                        0         67.9    76.5      84.6  16.7                                        15        80.2    84.5      88.0  7.8                                         100       81.1    83.2      85.3  4.2                                         ______________________________________                                         *NOTE: Balance nickel.                                                   

EXAMPLE 17

This example illustrates the beneficial effect of iodate ion addition toa palladium-nickel plating bath with no organic brighteners in promotingconstancy of alloy composition. Baths were formulated with the same lotof palladium diammine dichloride that was used in Example 16.Palladium-nickel alloy coatings were electrodeposited onto disks atcurrent densities ranging from 25 to 100 asf from baths containing 15and 100 ppm of iodate ions added as the sodium salt. The basic bathcomposition and operating conditions were as follows:

    ______________________________________                                        Bath Chemistry                                                                Pd concentration:                                                                             20 g/l as palladium                                                           diammine dichloride                                           Ni concentration:                                                                             10 g/l as nickel ammine                                                       sulfate                                                       Ammonium sulfate:                                                                             50 g/l                                                        Ammonium hydroxide:                                                                           quantity sufficient to                                                        achieve desired pH                                            Plating Conditions                                                            Temperature:    48° C.                                                 pH              8.5                                                           Speed of disk rotation:                                                                       500 rpm                                                       ______________________________________                                    

The Δ Wt% Pd.sub.(100-25) from the bath with the 15 ppm addition ofiodate ions was 7.6 and that for the bath with a 100 ppm addition ofiodate ions was 5.8. All coatings had a bright, mirror-like appearanceindicating the iodate ions were performing the function of a brighteneras well as that of a stabilizer of plated alloy composition.

While the above description and attached drawings illustrate certainembodiments of the present invention, it will be apparent that otherembodiments and modifications may be made that are equivalent thereto toone skilled in the art. For example, combinations of iodide and iodateion additions are also within the scope of this invention.

We claim:
 1. An electroplating bath for plating a coating of palladium-nickel alloy on an electrically conductive substrate and which is capable of maintaining the palladium content of the plated alloy substantially constant, said bath comprising a palladium salt, a nickel salt and at least about 15 parts per million of iodate ions.
 2. The bath of claim 1 further including a conductive salt and having a pH in the range between 7-9.
 3. The bath of claim 2 wherein the palladium salt is palladium (II) ammine chloride, the nickel salt is selected from the group consisting of nickel ammine sulfate, nickel sulfate and nickel chloride and the conductive salt is ammonium sulfate.
 4. The bath of claim 2 further including an organic brightener.
 5. The bath of claim 4 wherein the organic brightener is sodium vinyl sulfonate.
 6. The bath of claim 1, further including iodide ions.
 7. A process for plating a coating of palladium-nickel alloy on an electrically conductive substrate wherein the palladium content of the alloy remains substantially constant, said process comprising the steps of:(a) immersing the electrically conductive substrate in an electroplating bath comprising a palladium salt, a nickel salt and at least 15 parts per million of iodate ions; (b) applying plating current to the bath wherein the current density on the conductive substrate is in the range of about 10 amps/sq.ft. to about 150 amps/sq.ft; and (c) depositing a coating of palladium-nickel alloy on said substrate having a palladium content which varies less than 10 in weight percent over the current density range applied.
 8. The process of claim 7 wherein the current density on the conductive substrate is in the range of about 25 amps/sq.ft. to about 100 amps/sq.ft.
 9. The process of claim 8 wherein the current density on the conductive substrate may vary by as much as a factor of four.
 10. The process of claim 7 wherein the bath contains an organic brightener.
 11. The process of claim 7 wherein iodide ions are also present in the bath. 